1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) apparatus which is employed as a display apparatus incorporated in an information processing apparatus, an audio-video apparatus, an advertising indicator, etc. Furthermore, the present invention also relates to an electronic device for providing a control signal to the LCD apparatus.
2. Description of the Related Art
In recent years, personal computers and word processors have become widespread. As a display apparatus incorporated therein, a light-weight, thin LCD apparatus which may be driven by a battery has become dominant in the market replacing a CRT display apparatus which consumes a large amount of power and space.
A typical LCD apparatus employs a cold cathode ray tube as a light source for illuminating the display panel from the back, and the light source is driven by a light source driving circuit. The method for controlling the brightness in the display panel can generally be classified into two types of methods. One is a current-based light modulation method wherein the luminance is controlled by varying a current amplitude of the cold cathode ray tube. The other is a chopping light modulation method wherein the light source is alternately turned ON/OFF at a high speed based on a light modulation signal, and the luminance is virtually controlled by changing the duty ratio which is a ratio between an ON period and an OFF period of the light source.
The chopping method can accurately control the luminance over a wide brightness range. However, the frequency of flashing of the light used for the light modulation is apt to cause an interference with the driving frequency of the liquid crystal display device. As a result of the interference, flicker in the display and/or moving stripes are perceived on the screen.
In order to remove such drawbacks, various methods have been devised. Japanese Laid-Open Publication No. 4-143722 (xe2x80x9cPrior Art 1.xe2x80x9d) discloses a backlight and a control method thereof. As shown in FIG. 12, a light modulation signal generation circuit 21 is provided in a stage preceding a light source driving circuit 26 for driving a cold cathode ray tube 2. The frequency of the light modulation signal of the light source driving circuit 26 is adjusted so that the shift-number and a shift direction factor of light/dark portions on the screen are slightly offset.
Japanese Laid-Open Publication No. 3-64895 (xe2x80x9cPrior Art 2xe2x80x9d) discloses another control method for a backlight. A control device shown in FIG. 13 includes a duty control circuit 15 and a monostable multivibrator 11 coupled via an OR circuit 12 to a light source driving circuit 16 for driving a fluorescent lamp 2 which acts as a backlight of a liquid crystal display panel 1. The fluorescent lamp 2 is turned ON/OFF based on a rectangular wave signal whose duty cycle is variable. The rectangular wave signal is synchronized with a signal obtained by dividing a frequency of an image synchronization signal for a liquid crystal screen by an integer n (n greater than 0).
According to Prior Art 1 (Japanese Laid-Open Publication No. 4-143722), a light modulation signal frequency Fb and a driving frequency Ff of an LCD apparatus are set independent of each other. Therefore, even when a value suitable for the light modulation signal frequency Fb is determined by way of calculation, there remains a difficulty in regulating the driving frequency Ff and the light modulation signal frequency Fb with high accuracy while maintaining a desirable relationship therebetween. Furthermore, even when these frequencies are initially set to appropriate values, respectively, aging of the LCD device, temperature variation, etc., may cause a frequency-shift, whereby flicker in the display is perceived on the screen.
Prior Art 2 (Japanese Laid-Open Publication No. 3-64895) discloses a method for synchronizing the light modulation signal with the liquid crystal driving signal as shown in FIG. 13. However, regarding the frequency of the light modulation signal, Prior Art 2 only describes xe2x80x9ca signal corresponding to an image synchronization signal whose frequency is divided by an integer n equal to or greater than zero.xe2x80x9d Prior Art 2 fails to disclose specifically what type of signal the image synchronization signal is, and what number is optimal for the integer n. Therefore, even when a light modulation signal is obtained by suitably dividing the frequency of a horizontal synchronization signal having a driving cycle of one horizontal line or a display data latch pulse, which are used as liquid crystal driving signals, flicker in the display may be generated in many cases.
According to one aspect of the present invention, a liquid crystal display apparatus includes a liquid crystal display device: a light source for illuminating the liquid crystal display device; a LCD device driving circuit for providing a driving voltage to the liquid crystal display device based on a display data signal and a plurality of liquid crystal driving signals including a display data latch signal; a dividing circuit for dividing a frequency of the display data latch signal by a factor of N so as to obtain a period which is N times as large as that of the display data latch signal, where N is an integer greater than zero; a duty control circuit for changing an ON duty ratio of the frequency-divided signal by using the frequency-divided signal as a reference frequency; and a light source driving circuit for turning the light source ON/OFF based on a signal from the duty control circuit having the ON duty ratio set in the duty control circuit, wherein, where a driving duty is (1/D), and a remainder of D divided by the integer N is A, each of the integer N and the value D is set to an integer greater than zero which satisfies the following expression:
xe2x88x921xe2x89xa6{(N/A)xe2x88x922}xe2x89xa61.
According to another aspect of the present invention, a liquid crystal display apparatus includes a liquid crystal display device; a light source for illuminating the liquid crystal display device; a LCD device driving circuit for providing a driving voltage to the liquid crystal display device based on a display data signal and a plurality of liquid crystal driving signals; a dividing circuit for dividing a frequency of a horizontal synchronization signal having a period of one horizontal period, to obtain a period which is N times as large as that of the a horizontal synchronization signal, where N is an integer greater than zero; a duty control circuit for changing an ON duty ratio of the frequency-divided signal by using the frequency-divided signal as a reference frequency; and a light source driving circuit for turning the light source ON/OFF based on a signal f rom the duty control circuit having the ON duty ratio set in the duty control circuit, wherein where a period of the vertical synchronization signal is M times as large as a period of the horizontal synchronization signal, and a remainder of M divided by N is A, each of the integer N and the value M is set to an integer greater than zero which satisfies the following expression:
xe2x88x921xe2x89xa6{(N/A)xe2x88x922}xe2x89xa61.
According to still another aspect of the present invention, a liquid crystal display apparatus includes a liquid crystal display device; a light source for illuminating the liquid crystal display device; a LCD device driving circuit for providing a driving voltage to the liquid crystal display device based on a display data signal and a plurality of liquid crystal driving signals including a display data latch signal; a dividing circuit for dividing a frequency of the display data latch signal or a horizontal synchronization signal having a cycle of one horizontal period, to obtain a period which is M times as large as that of the display data latch signal or the horizontal synchronization signal, where M is an integer greater than zero; a duty control circuit for changing an ON duty ratio of the frequency-divided signal by using the frequency-divided signal as a reference frequency; and a light source driving circuit for turning the light source ON/OFF based on a signal from the duty control circuit having the ON duty ratio set in the duty control circuit, wherein, where a period of the display data latch signal or the horizontal synchronization signal is tLP, and a rising edge of a light modulation signal with respect to the beginning of a frame period is offset by (Bxc3x97tLP), the value B is set to an integer greater than zero which satisfies the following expression:
xe2x88x921xe2x89xa6{(M/B)xe2x88x922}xe2x89xa61.
According to still another aspect of the present invention, a liquid crystal display apparatus includes a liquid crystal display device; a light source for illuminating the liquid crystal display device; a LCD device driving circuit for providing a driving voltage to the liquid crystal display device based on a display data signal and a liquid crystal driving signal having a frame period T1; a duty control circuit for changing an ON duty ratio of a signal having a period T2; a light source driving circuit for turning the light source ON/OFF based on a signal having the ON duty ratio set in the duty control circuit, wherein, where a remainder of T1 divided by T2 is A, the division-number is set to an integer greater than zero which satisfies the following expression:
xe2x88x921xe2x89xa6{(T2/A)xe2x88x922}xe2x89xa61.
According to still another aspect of the present invention, an electronic device for providing a control signal for use in a liquid crystal display apparatus, wherein, where a driving duty ratio is 1/D, a division-number of a display data latch signal to be used in the liquid crystal display apparatus is an integer N, and a remainder of D divided by the division-number N is A, each of the division-number N and the value D is set to an integer greater than zero which satisfies the following expression:
xe2x88x921xe2x89xa6{(N/A)xe2x88x922}xe2x89xa61.
According to still another aspect of the present invention, an electronic device for providing a control signal for use in a liquid crystal display apparatus, wherein, where a division-number of a horizontal synchronization signal having a cycle of one horizontal period is an integer N, a cycle of a vertical synchronization signal to be used in the liquid crystal display apparatus is M times as large as that of a horizontal synchronization signal, and a remainder of the value M divided by the division-number N is A, each of the division-number N and the value M are set to an integer greater than zero which satisfies the following expression:
xe2x88x921xe2x89xa6{(N/A)xe2x88x922}xe2x89xa61.
According to still another aspect of the present invention, an electronic device for providing a control signal for use in a liquid crystal display apparatus, wherein, where a period of a display data latch signal or a horizontal synchronization signal having a cycle of one horizontal period to be used in the liquid crystal display apparatus is tLP, and a rising edge of a light modulation signal with respect to the beginning of a frame period is offset by (Bxc3x97tLP), the value B is set to an integer greater than zero which satisfies the following expression:
xe2x80x83xe2x88x921xe2x89xa6{(M/B)xe2x88x922}xe2x89xa61.
According to still another aspect of the present invention, an electronic device for providing a control signal for use in a liquid crystal display apparatus, wherein, where a frame period of the liquid crystal display apparatus is T1, a signal having an ON duty ratio to be changed has a period T2, and a remainder of Ti divided by T2 is A, the electronic device outputs the control signal while setting a division-number to an integer greater than zero which satisfies the following expression:
xe2x88x921xe2x89xa6{(T2/A)xe2x88x922}xe2x89xa61.
Hereinafter, functions of the present invention will be described.
According to the above-described structure, the frequency of a display data latch signal for driving the liquid crystal display device or the frequency of a horizontal synchronization signal having a cycle of one horizontal period is divided by division-number N by a dividing circuit, and the frequency-divided signal is output to a duty control circuit. The duty control circuit changes the ON duty ratio of the frequency-divided signal, and outputs a signal having a modified duty ratio to a light source driving circuit. Based on the signal from the duty control circuit, the light source driving circuit drives the light source for illuminating the liquid crystal display device.
Herein, assuming that (1/D) is a driving duty ratio of the LCD apparatus, xe2x80x9cAxe2x80x9d is the remainder of D/N, and N is set so as to satisfy the following condition:
xe2x88x921xe2x89xa6{(N/A)xe2x88x922}xe2x89xa61.xe2x80x83xe2x80x83(Condition 1)
Then, the light source is turned ON/OFF for each frame (i.e., with the frame frequency). The frame frequency is typically equal to or greater than 60 Hz. With such a high frequency, flicker in the display is not perceived by the human eye because the cycle of flashing is extremely fast. Accordingly, light for the display can be appropriately modulated.
Furthermore, even in the case where the frequency of the LCD device driving signal fluctuates due to aging of the LCD device and/or temperature variation, the light modulation signal synchronizes with the fluctuated frequency because the light modulation signal is obtained by dividing the frequency of a liquid crystal display device driving signal. Thus, a stable driving operation can be carried out independent of such frequency fluctuation. Accordingly, the light for the display is consistently maintained in a suitable state while flicker in the display is reduced.
Furthermore, the dividing circuit divides the frequency of a display data latch signal or the frequency of a horizontal synchronization signal having a period of xe2x80x9ctLPxe2x80x9d by a division-number M. A rising edge of the light modulation signal is offset with respect to the beginning of a frame period by (Bxc3x97tLP) for each frame. The value B is set so as to satisfy the following expression:
xe2x88x921xe2x89xa6{(M/B)xe2x88x922}xe2x89xa61.
Thus, the light source is turned ON/OFF for each frame based on a frame frequency in a manner similar to that under condition 1.
Thus, for the same reasons as those described above, flicker is not perceived by the human eye while the light can be appropriately modulated. Furthermore, even in the case where the frequency fluctuates due to aging and/or temperature variation, a stable driving can be performed without being affected by such a frequency fluctuation.
Especially in the case of frequency fluctuation, the value B can be set independent of the relationship between the division-number M and the driving duty ratio. Accordingly, flicker in the display can be reduced under a wider variety of conditions.
Thus, the invention described herein makes possible the advantage of providing an LCD apparatus capable of suppressing the generation of flicker in the display and capable of maintaining a stable display with reduced flicker even when the driving frequency and/or the light modulation signal frequency fluctuate due to aging of the LCD apparatus and/or temperature variation.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.